1. Field of the Invention
The present invention relates generally to the field of liquid feed fuel cells, including direct oxidation fuel cells and, more particularly, to a fuel container and delivery apparatus for systems including such fuel cells.
2. Background Information
Fuel cells are devices in which an electrochemical reaction is used to generate electricity. A variety of materials may be suited for use as a fuel depending upon the materials chosen for the components of the cell. Organic materials, such as methanol or natural gas, are attractive choices for fuel due to the their high specific energy. Liquid feed fuel cells employ a liquid substance, such as methanol, as the fuel.
By way of background, fuel cell systems may be divided into “reformer-based” systems (i.e., those in which the fuel is processed in some fashion to extract hydrogen from the fuel before it is introduced into the fuel cell) or “direct oxidation” systems in which the fuel is fed directly into the cell without the need for separate internal processing. Most currently available fuel cells are reformer-based fuel cell systems. However, because fuel-processing is technically complex, difficult and requires significant volume, reformer based systems are presently limited to comparatively high power applications.
It should be understood that the fuel used in the cell may be either a carbonaceous liquid or a gas. A fuel cell that utilizes a liquid fuel is said to be a “liquid feed” fuel cell. A liquid feed fuel cell may be further categorized as a “liquid feed reformer-based fuel cell” or a “liquid feed direct oxidation fuel cell”. In some instances, it may be desirable to store and utilize a liquid fuel, rather than a gaseous fuel, due to the ease of handling and storage of liquids, and comparative stability of a liquid under a wide range of environmental conditions. It should also be understood that this description is related primarily to liquid feed fuel cell systems, and as such the systems are categorized simply as direct oxidation or reformer-based systems.
In lower power operations, such as hand held portable electronics, it may be advantageous to utilize a direct oxidation fuel cell system. More specifically, direct oxidation fuel cell systems may be best suited for a number of applications in smaller mobile devices (e.g., mobile phones, handheld and laptop computers), as well as in some larger applications.
Briefly, in direct oxidation fuel cells, a carbonaceous liquid fuel (typically in an aqueous solution such as an aqueous methanol solution) is introduced to the anode face of a membrane electrode assembly (MEA). The MEA contains a protonically-conductive but, electronically non-conductive membrane (PCM). Typically, a catalyst, such as platinum or a platinum/ruthenium alloy, which enables direct oxidation of the fuel on the anode is disposed on the surface of the PCM (or is otherwise present in the anode chamber of the fuel cell). Protons (from hydrogen found in the fuel and water molecules found on the anodic face of the reaction) are separated from the electrons. The protons migrate through the PCM, which is impermeable to the electrons. The electrons thus seek a different path to reunite with the protons and oxygen molecules involved in the cathodic reaction. Accordingly, the electrons travel through a load, providing electrical power.
One example of a liquid feed fuel cell system is a direct oxidation fuel cell system, and more specifically, a direct methanol fuel cell system (or “DMFC” system). In a DMFC system, methanol in an aqueous solution is used as the liquid fuel (the “fuel mixture”), and oxygen, preferably from ambient air, is used as the oxidizing agent. There are two fundamental reactions that occur in a DMFC which allow a DMFC system to provide electricity to power-consuming devices: the anodic disassociation of the methanol and water fuel mixture into CO2, protons, and electrons; and the cathodic combination of protons, electrons and oxygen into water.
In order for these reactions to proceed continuously, fuel cells, including liquid feed fuel cells, must be supplied with sufficient fuel to ensure power generation. Moreover, if such a liquid feed fuel cell is to be used with a portable, handheld device, it ideally should operate, effectively, in a variety of orientations. Accordingly, a DMFC, when used in a portable electronic device should include a fuel delivery system that delivers liquid fuel on either a continuous basis or upon demand, regardless of the orientation of the DMFC system.
Due to the nature of methanol, and its associated risks to persons and properties, safety precautions are typically followed when using this substance. It is thus desirable to store and deliver methanol in a manner that substantially prevents leakage of the fuel from the container. Furthermore, the fuel substance may be mixed with one or more additives that increase its detectability in case it does escape from its container. These safety enhancing additives allow for safer handling of the fuel substance by providing an odor and/or color to increase the likelihood of detection of the substance, by a person who may come in contact with it if amounts of methanol are released from the fuel cell, either upon disposal or accidental breakage.
For best results, the safety-enhancing additives should be stored and maintained separately from the fuel while the fuel is in use powering the relevant device. In addition to safety enhancing additives, there are effluent substances that are produced in the fuel cell reactions that may or may not be useful. For example, on the anode aspect of the fuel cell, carbon dioxide is a product of the reaction, and there may also be excess or un-reacted fuel, water, and other products of the reaction or substances present. On the cathode side, water is produced, which may be removed, and additionally, other substances or contaminants may also be present that are desired to be removed. It may be desirable to remove some of these substances in a convenient manner.
The device should also conform to a small form factor and these advantages should be provided at an expense level that allows mass manufacturing techniques to remain feasible. Accordingly, it is an object of the invention to provide a storage container and delivery system that feeds liquid fuel to a fuel cell in a continuous, or periodic manner, but without unexpected interruption even while the device (being powered by the fuel cell) is operated in a variety of orientations, and which removes unwanted effluent from the fuel cell.